A tendency exists in some applications like server or telecom power systems towards increasing a power density in the systems by increasing a required output power of a power supply while keeping the same or even reducing the power supply form factor. As a consequence, more efficient power supplies are required. Furthermore, a high efficiency of such power supplies is needed over a wide load range including light load operation, for example in sleep mode or other low power mode of systems.
For such applications, resonant converters are increasingly used. Resonant converters are a type of electric power converters that contain a network of inductors and capacitors referred to as “resonant tank”, tuned to resonate at a specific frequency. One specific type of resonant converters are so-called LLC converters, where the resonant tank includes a series connection of two inductors, one of them being a primary winding of a transformer.
In high output current applications, i.e. applications where a high output current is required at least during some times such as server and telecom power supply systems providing a plurality of power supplies in parallel is a common practice. In particular, in such applications more than one resonant converter may be used in parallel. Often the resonant converters of such an arrangement are controlled in an interleaved manner by applying phase delays in signals controlling the converters. In this way, currents output by individual converters of a system are phase shifted with respect to each other. This may provide advantages such as reduced current stress, loss distribution among the converters, easier thermal management, heat sink reduction or output current ripple reduction. Because of the output current ripple reduction, a size of an output filter usually used for such converter systems may be considerably reduced compared to a case without interleaving.
Furthermore, in such systems so-called phase shedding techniques may be used, which essentially means that in light load conditions one or more of the converters are deactivated, and only the remaining converters continue to operate. This may lead to reducing current consumption under light load conditions.
Resonant converters typically use the switching frequency as a control parameter in order to modify gain characteristics of the resonant tanks, for example to thereby adjust an output voltage of the resonant converter. In arrangements where a plurality of such converters are used, and to implement the above-mentioned interleaving, all of the resonant converters usually need to operate at the same switching frequency, and phase delays are applied between control signals having this switching frequency for different converters.
Resonant tanks of such converters are manufactured with a manufacturing tolerance. Operation of the tanks of different converters with the same switching frequency, due to such tolerances, may lead to a poor current balance among the converters, i.e. one converter providing more power than another converter, which in a worst case can lead to one of the converters handling most of the power.
In this case, overcurrent or overpower protections may be triggered, thus turning off the converter, or the converter may be damaged due to elevated current stress.
For example, in some LLC converters according to simulations a 3% variation in parameters of the resonant tank (inductivity of inductors and capacitances of capacitors) may lead to a current imbalance of 65% compared to a nominal output current for each converter.
Various solutions have been proposed for this problem, including application of separate current loops or using a common inductor. Other techniques include providing adjustable elements (inductors and/or capacitors) in the resonant tank or providing gain boosts by effectively short circuiting a secondary side of a transformer used in the converter.
These conventional approaches have various drawbacks like not being able to use interleaving techniques, additional area requirements for adjustable components or an inability to use switching frequencies at or above a resonance frequency of the resonant tank.